07 January 2011

Lowest of Lows

If you read home and garden magazines, shop for mail-order seeds, or listen to programs like Wisconsin Public Radio’s Garden Talk, you are probably familiar with the “zone” system. If not, here are the basics:

Not all plants can survive in all climates. At a certain point a plant will freeze to death. Similarly, as the mercury rises, there is a point at which a plant or vine will simply bake and wither. Thus, a map of climatic zones was established to help gardeners and landscapers know (a) what zone they are working in and (b) which plants, trees, and flowers can survive there. The zones are labeled with low numbers for colder climates and high numbers for heat. The continental United States covers, roughly, zones 2 (in Montana, North Dakota and Minnesota) through 10 (in southern Florida and California’s Death Valley).

It is a fairly simple concept and the resulting map looks something like this.


I’ve consulted zone maps many times. They are very helpful. But, being the inquisitive, ornery type, I wondered, Where does this information come from? Who determines these zones?

I had my theories. I thought, perhaps, it was a product of the gardening industry – something developed by retailers to help customers make informed choices and avoid refund requests when orchids start dying off in February in Fargo. Or, maybe it was compiled by an organization like the American Horticultural Society as a sort of public service announcement. I was curious.

But, with a little digging, I learned a lot more about the zone map than I expected.


Its official name is the United States Department of Agriculture Plant Hardiness Zone Map. That’s right. This helpful little tool is actually product of the oft-maligned federal government and its even more maligned bureaucrats and researchers. Unlike other maps associated with the US government, though, it offers a great deal more insight and nuance than the red and blue, either/or maps used during the now-perpetual American election cycle. It also, in a refreshing break from our national obsession with ourselves, includes more than the fifty states.

The current map, produced by the National Arboretum (a branch of the USDA) in Washington D.C., was published in 1990 and covers most of North America. It replaced the 1965 edition of the map and looks like this.


Authored by then-Director of the National Arboretum, Henry M. Cathey, the 1990 Hardiness Map includes eleven primary zones (1-11), most of which are further divided into subzones (e.g., zones 4a and 4b). The result is a grand total of twenty distinct hardiness zones stretching from Mexico's southern border to the arctic expanses of northern Canada.

Betcha didn’t know that. Yeah, neither did I.

(I should note that other organizations such as the Arbor Day Foundation have compiled similar – sometimes more recent – hardiness maps. They also produce maps showing temperature changes over time. By most measures, however, the USDA map is still the map of record.)

Even with this interesting background, the most intriguing and surprising features of the map are its simplicity and specificity.

Originally, I had the impression that the map grew from a complex assessment of multiple factors such as temperature, soil, and rainfall. Nope. The map is based entirely on temperature. Knowing this, I assumed the zones were based on temperature ranges. Wrong again. The Hardiness Map is based only on low temperatures. Finally, learning that low temps were the key, I figured the map represented average low temperature. Strike three. Each zone – this is the really neat part – is defined by the average lowest temperature.

O-o-o-kay. What’s the difference? Who cares?

When most of us imagine “average low temperature,” we think about how cold it gets in general. This is pretty easy to calculate and gives us the sort of information we care about in terms of comfort and wardrobe choice. Consider the average low temperature last month. All you need to do is take the lowest temperature for each day, add them up, and divide by the number of days in December, thirty-one.

Dec. 1 Low + Dec. 2 Low + Dec. 3 … + Dec. 31
Divide by 31 days

This will give you a reasonable indication of how cold it was in December 2010. Some days were colder than average and some warmer. But you get the picture. If you wanted to know about Decembers (plural), you could average this information again over the course of a decade. Simply add together the average December lows from 2000 to 2010 and divide by ten years. Easy peasy.

The USDA map, however, provides slightly different information. The map will not tell you how cold it is generally. Instead, it shows how cold it gets on the coldest of cold days.

The 1990 version of the map was developed with data from 1974 to 1986, so the equation would like this:

Lowest Temp in 1974 + Lowest in 1975 + 1976… + 1986
Divide by 12 years

The math is the same, but the result is very different. Unlike the first equation, which gives the average low, this formula will provide you with the average lowest temperature.

See the difference?

Ok. I’ll stop with the math. (I don’t want to be burdened with the guilt of having induced PTSD flashbacks of 9th grade algebra.) Suffice it to say, the zone map is not – as I had presumed – a general guide based on a collection of diverse information. It is actually a very precise source of specific information.

It is not all the information a gardener needs, of course. The National Arboretum reminds map users to consider other factors such as stress (e.g., pollution), plant management systems (e.g., new technologies for transplanting and transporting plant life), and artificial environments (e.g., planting in a spot, like an elevated deck, where the plant will be “separated from the ground and its warming influence”). Soil type and rainfall are also relevant variables. But this particular piece of data, average minimum temp, is indispensable nonetheless.

Unless you are prepared to undertake dramatic measures to protect and warm your plants, you do not want to know how cold it is generally. You want to know the coldest temperature possible. Why? As many disappointed gardeners have learned (some of us have had to learn this multiple times) it only takes one fatefully frigid night to decimate a tomato plant, peach tree, or rose bush. That is, your peas aren’t going to care about average temperature for the whole month or year when they are freezing their pods off right now.

That, friends, is the difference.

As for who cares? Every gardener cares whether he plants something with little-to-no hope of surviving the night’s chill. Avoiding this worse-case scenario is the whole point behind hardiness zones. But, I suppose your real question is: If the map works, who cares how we got it?

Well, I thought it was interesting.

In the mean time, folks, enjoy the winter. Don't begrudge the ice and snow too much. Somewhere between March and May, spring will arrive – no matter what your zone is – and there will be a fresh, new chance to grow again.

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